1. Field of the Invention
The present invention relates to a multi-cylinder internal combustion engine wherein each cylinder is capable of operating with controlled auto-ignition within a predetermined torque/speed range.
2. Background of the Invention
It is possible to operate homogeneous charge internal combustion engines under conditions in which controlled auto-ignition of the charge occurs. This mode of engine operation results in reduced NOx emissions, and improved driveability and fuel economy, and is particularly beneficial during low torque conditions.
One way of achieving controlled auto-ignition is to retain a high proportion of hot residual gases in the combustion chambers. When a new premixed charge is admitted, the residual gases serve to raise the temperature in the combustion chamber to the point where the fuel in the premixed charge ignites spontaneously. When this occurs, combustion commences simultaneously at multiple ignition sites giving a fast burn and even temperature. The even temperature is believed to be responsible for low generation of NOx and the fast burn results in high thermal efficiency.
There is however a minimum ratio of residual gases to fresh intake charge below which auto-ignition cannot occur. This minimum ratio defines the upper torque limit of the auto-ignition range. When torque is low, a high ratio of residual gases to fresh intake is required for auto-ignition to occur. However, with reducing torque, the amount of fuel burned during each cycle is reduced accompanied by a consequent drop in combustion temperature. Eventually a point is reached where the residual gases are too cold to bring about auto-ignition and this defines the lower torque limit of the auto-ignition range.
Under certain conditions, it is possible to operate an engine in auto-ignition mode for a substantial proportion of its running time but nevertheless there may be occasions, such as idling and steady low torque operation, when auto-ignition could yield significantly reduced fuel consumption but these conditions lie below the lower torque limit of the auto-ignition range.
The present invention therefore seeks to increase the proportion of running time that an engine can operate with auto-ignition when operating under low to medium torque conditions.
The present invention therefore seeks to increase the proportion of running time that an engine can operate with auto-ignition when operating at low to medium torque.
According to the present invention, there is provided a method for controlling a multi-cylinder internal combustion engine comprising disabling selected cylinders when an average cylinder torque lies below an auto-ignition torque/speed range so as to raise an output torque of the remaining active cylinders and enable them to continue to operate within said auto-ignition torque/speed range.
In order for the torque to remain constant after cylinder disablement, it is essential to increase the charge supplied to the remaining cylinders. In so doing, the operating cylinders are producing higher torque, bringing them within the auto-ignition range. In this way, the invention allows the torque/speed range within which the whole engine may benefit from controlled auto-ignition to be extended to include lower engine torques.
The manner in which the charge to the active cylinders is increased will depend on the manner in which mass airflow is controlled. If the engine has an intake throttle, then the throttle position and the fuel delivered to the active cylinders may be increased in synchronism with the disablement of the cylinders so that there should be no perceptible change in output power. Similar synchronization can also be achieved in an engine in which mass airflow is controlled by the use of variable valve timing.
It is possible to disable selected cylinders in a variety of known ways. For example, it would suffice to disable the fuel supply to the cylinder by deactivating the intake and exhaust valves.
From mapping the operation of the engine, it is possible to determine the torque/speed range within which auto-ignition can be achieved. Hence, it is possible in one embodiment of the invention to disable selected cylinders when the engine is sensed to be operating below this range.
Alternatively, it is possible to measure exhaust gas temperature directly and to resort to cylinder disablement when the temperature drops below a first lower threshold. If the engine is operating with a reduced number of cylinders, all cylinders can again be rendered operative when the exhaust temperature of the working cylinders is sensed to exceed a second higher threshold.